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Hydrogen storage capacity on different carbon materials
Chemical Physics Letters 485 (2010) 152–155
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Hydrogen storage capacity on different carbon materials Vicente Jiménez a,*, Paula Sánchez a, José Antonio Díaz a, José Luis Valverde a, Amaya Romero b a b
Facultad de Ciencia Químicas/Escuela Técnica Agrícola, Departamento de Ingeniería Química, Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain Facultad de Ciencia Químicas/Escuela Técnica Agrícola, Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain
a r t i c l e
i n f o
Article history: Received 6 November 2009 In final form 9 December 2009 Available online 18 December 2009
a b s t r a c t The hydrogen adsorption capacity of various carbon structures including activated carbon (AC) and carbon nanofibers (CNFs) has been measured as a function of pressure and temperature. Results have shown the correlation between hydrogen storage capacity and specific micropore surface area and also explain the hysteresis in H2 uptake observed at 273 and 299 K principally with CNFs materials. The highest H2 storage value (2.02 wt%) was obtained with AC at 10 bar and 77 K. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Hydrogen, as a futuristic energy resource free of air pollution and greenhouse effects (its combustion does not generate pollutants such as particles, nitrogen oxides, sulfur oxides, hydrocarbons and carbon monoxide), has attracted a great deal of attention in recent years. Thus, hydrogen storage is widely recognized as a critical enabling technology for the successful commercialization and market acceptance of hydrogen powered vehicles. The individual parts of hydrogen energy system such as production, delivery, storage and conversion are closely interrelated. Among them, hydrogen storage is one of the bottlenecks for the applications in automobile. Several storage methods have been investigated to develop efficient technologies [1]. Hydrogen storage by gas compression, low temperature liquefaction, metal hydride formation has several disadvantages. None of the current hydrogen storage technology satisfies all of the needs requested by end users and producers. Therefore, new investigations are centred in the development of novelty hydrogen storage materials. In this sense, it has been demonstrated that carbonaceous materials are attractive candidates due to their adsorption ability, high porosity, low mass density and low cost. In this work has been presented experimental results of hydrogen adsorption at different temperatures and pressures over activated carbon, carbon nanofibers and activated carbon nanofibers. Hydrogen adsorption data are interpreted according to the properties of the carbon materials (specific surface area, pore size distribution, etc.) and to the adsorption conditions (T, P).
2.1. Preparation and activation of the adsorbents
* Corresponding author. E-mail address: [email protected] (V. Jiménez). 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.12.026
Herringbone type carbon nanofibers were grown at atmospheric pressure in a fixed-bed reactor located in a vertical oven at a temperature of 600 °C according to the procedure described in [2,3]. Activated carbon was obtained from PANREAC. AC samples received the same HF treatment than synthesized CNFs for comparison purposes. CNFs were activated in an experimental set up consisting on a horizontal quartz reactor tube with a conventional horizontal furnace. Thus, the appropriate amount of CNFs was mixed with KOH and distilled water (10 ml water for 2 g KOH) and heated at 85 °C for 4 h under stirring and then dried for 12 h at 110 °C. Finally, the mixture was placed on a ceramic crucible located inside the horizontal reactor tube. The heat treatment consisted of a heating ramp from ambient temperature to 850 °C at a heating rate of 5 °C/min, followed by a 3 h plateau under 700 ml min1 He flow rate. Later, the system was cooling back to the initial temperature. The activated product (A-CNFs) was firstly washed with hydrochloric acid (5 M) to remove the activating agent and finally, with distilled water until a neutral washing was obtained. The resulting material was dried for 12 h at 110 °C in air to remove water prior to characterization.
2.2. Adsorbents characterization Surface area/porosity measurements were carried out using a Micromeritics ASAP 2010 sorptometer apparatus with N2 at 77 K as the sorbate. The samples were outgassed at 453 K under vacuum (6.6 109 bar) for 16 h prior to analysis; specific surface areas were determined by the multi-point BET method, pore geometry and size distributions were evaluated using the standard BJH
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V. Jiménez et al. / Chemical Physics Letters 485 (2010) 152–155
treatment and micropore size distribution were evaluated using the Horvath–Kawazoe (H–K) equation. XRD analyses were carried out on a Philips X’Pert instrument using nickel filtered Cu Ka radiation; the samples were scanned at a rate of 0.02° step1 over the range 5 6 2h 6 90° (scan time = 2 s step1). This technique was used to evaluate the crystallinity of the carbon nanofibers.
cropore: 0.7–2 nm and ultramicropores: <0.7 nm); 2 nm < mesopore <50 nm and, macropore >50 nm. Obviously, AC sample presented the highest ultramicropore amount, followed by the A-CNFs sample. It can be observed that, the chemical activation process produced principally an increase in the ultramicropore volume (coincident with the BET surface area increase) although some supermicropores and mesopores were also generated. Note that although the quantity of mesopores increased as consequence of the activation, the percentage of mesoporosity inherent to the parent CNFs decreased, due to the high amount of ultramicropores created. Regarding to the pore development after an activation process, it is important to comment that different pore opening mechanisms exist. Whereas the micropore development is correlated with the partial opening of graphite layers in CNFs (d002 increase), the mesopore development is probably related to others behaviours such as, breaking of nanofibers or full exfoliation of graphite layers which can cause different sizes of mesopores according to the degree of exfoliation (even in the collapse of some micropore structures) [3]. The CNFs graphitization degree changes after the activation process can be quantified by means of npg values. After the activation, peaks corresponding to hexagonal (2h = 26°) and rhombohedral graphite (2h = 45°) become wider by treatment of CNFs with KOH (XRD patterns not showed), indicating that these CNFs were less graphitic in nature [2,3]. As consequence, d002 and Lc values increased and decreased respectively, being npg values lower than that obtained before the activation. Nevertheless, these npg values were pretty high if compared with those reported for conventional activated carbon or activated carbon fibers indicating that graphene layers remained undestroyed, and activation results in violation of parallel packing of layers (besides of burning some less ordered carbon) [5]. Table 2 shows the hydrogen adsorption at different temperatures and pressures obtained with the different carbon materials. Likewise, the variation of hydrogen uptake (mmol H2/g) at different temperatures and 10 bar of pressure are shown in Fig. 2. In the adopted experimental conditions, the following order in H2 uptake was observed: AC > A-CNFs > CNFs. With all carbon structures, hydrogen adsorption at 77 K resulted to be completely reversible, with almost no hysteresis phenomena and all isotherms having very similar shape. Nevertheless, it could be observed hysteresis in the adsorption–desorption kinetics above the supercritical temperature of H2, principally with CNFs materials. This behaviour
2.3. Hydrogen storage measurements Hydrogen adsorption capacities of carbon materials were measured using a Micromeritics ASAP 2050 (estimated error measurement ± 2%). The samples were outgassed at 453 K under vacuum (6.6 109 bar) for 16 h prior to analysis. The isotherm adsorption was obtained until 10 bar of absolute pressure to different temperatures. H2 storage at P > 10 bar was measured using a volumetric apparatus as explained in [4]. Prior to analysis, sample inserted into the cell was evacuated for several hours to a pressure level below 109 bar. 3. Results and discussion Table 1 summarizes the textural properties of the carbon materials as well as the principal XRD parameter, npg (mean number of grapheme planes in the crystallites). On the other hand, the proportion of ultramicropores, supermicropores and mesopores in the carbon samples are shown in Fig. 1. The following classification of the pores (IUPAC) was considered: micropore <2 nm (supermi-
Table 1 Textural properties and npg values of the carbon materials. AC
A-CNFs
CNFs
884 806 (91%) 17.276 (98%) 0.285 1.55
570 305 (54%) 3.431 (75%) 1.149 6.70
127 33 (26%) 0.147 (20%) 0.590 18.80
a
In brackets: percentage of micropore area with respect to the total surface area. Cumulative micropore volume obtained using the Horvath-Kawazoe method. In brackets: percentage of micropore volume with respect to the total pore volume. c Cumulative mesopore volume obtained using the BJH method. d Calculated using the formula npg = Lc/d002, where Lc is the average stacking height of carbon planes and, d002 is the average interlayer spacing. b
4.0
18
94 %
Parent CNFs Activated CNFs Activated carbon
3.5 66 %
14 12
2.5
3
3
Pore volume (m /g)
3.0
16
Pore volume (m /g)
BET surface area (m2/g) Micropore areaa (m2/g) Micropore volumeb (cm3/g) Mesopore volumec (cm3/g) npgd
10 2.0 8 1.5
25 %
1.0
6 4
9%
0.5 26 %
12 %
4%
62 %
2 2%
0.0
0 Ultramicropores
Supermicropores
Mesopores
Fig. 1. Proportion of ultramicropores (<0.7 nm), supermicropores (0.7–2 nm) and mesopores (2–50 nm) of the carbon materials (IUPAC classification).
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V. Jiménez et al. / Chemical Physics Letters 485 (2010) 152–155
Table 2 Hydrogen storage data of carbon materials. Pressure: 10 bar Temperature: 77 K mmol H2/g
wt%
Influence of the adsorption temperature AC 10.1 2.02 A-CNFs 5.0 1.00 CNFs 1.6 0.30 Pressure: 10 bar Temperature: 299 K Influence of the adsorption pressure AC 0.5 0.10 A-CNFs 0.2 0.05 CNFs 0.1 0.02
Pressure: 10 bar Temperature: 273 K mmol H2/m2
mmol H2/g
0.0130 0.0200 0.0500
0.0006 0.0008 0.0030
Pressure: 10 bar Temperature: 299 K mmol H2/m2
mmol H2/g
0.5 0.10 0.4 0.08 0.2 0.04 Pressure: 25 bar Temperature: 299 K
0.0006 0.0013 0.0060
0.5 0.10 0.2 0.05 0.1 0.02 Pressure: 35 bar Temperature: 299 K
0.0006 0.0008 0.0030
2.6 1.4 1.0
0.0030 0.0050 0.0300
3.2 2.0 1.4
0.0040 0.0080 0.0400
wt%
0.51 0.29 0.20
0.7 AC A-CNFs CNFs
10
0.6
2
Adsorbed hydrogen (mmol/g)
4
AC A-CNFs CNFs
273 K
0.4 Adsorbed hydrogen (mmol/g)
Adsorbed hydrogen (mmol/g)
6
0.65 0.40 0.28
mmol H2/m2
0.5 AC A-CNFs CNFs
77 K
8
wt%
0.5
0.4
0.3
0.2
299 K
0.3
0.2
0.1
0.1 0
0.0
0.0 0
2 4 6 8 10 Absolute pressure (bar)
0
2
4
6
8
Absolute pressure (bar)
10
0
2 4 6 8 10 Absolute pressure (mmHg)
Fig. 2. H2 adsorption–desorption isotherms at different temperatures and 10 bar for the carbon materials.
would allow H2 to be adsorbed at high pressures but stored at lower pressures. The differences in isotherm hysteresis must be related to the detailed characteristics of each porous structure [6]. CNFs and A-CNFs had the most marked isotherm hysteresis due to their smallest windows dimensions (d002) (similar to the 2.89 Å kinetic diameter of H2) combined with cavities(pores) that are much larger than H2 [3]. This fact impeded H2 access and at consequence, desorption of the trapped H2 guest to occur via window opening, which became noticeable at very low temperatures. As can be observed, at 299 and 273 K the hydrogen uptake (adsorption branch) was a linear function of the pressure indicating that no saturation in the investigated pressure range and that the adsorbed hydrogen layer on the carbon surface was very diluted. Nevertheless, at low temperatures (77 K) the isotherms can be fitted with a Langmuirtype equation (type I isotherm) indicating that saturation takes place with a hydrogen monolayer formation, as usual for microporous surfaces. In the absence of pore condensation effects (T > Tc) and relatively low P, adsorbents that have narrower pores (AC) maximize the interaction potential between hydrogen molecules and carbon adsorbents because of overlap of the potential fields from both sides of the pore, and lead to pore filling and superior performance at low P. Nevertheless, larger pores (CNFs/A-CNFs) allow increasing uptake at high P. Finally, notice that the hydrogen coverage per unit of micropore surface area at any pressure and/ or temperature was larger on nanostructures than on activated
carbon (last column of Table 2), which suggests that an increase of the surface area of these materials could possibly lead to interesting hydrogen storage capacities. As can be observed, excepting AC at 77 K, all the other experimented carbon materials did not allow any significant hydrogen storage. In spite of obtained results were very low, data were comparable with those of literature (see Table 3). In December 1996, Rodríguez and Baker claimed that their carbon nanofibers can store up to 70 wt% of hydrogen at room temperature and 140 bar hydrogen pressure. In recent years, different groups tried to reproduce the results of Rodriguez and Baker in similar or modified carbon nanostructures. It was claimed that alkali-doped carbon nanotubes can store up to 20 wt% of hydrogen, which later was demonstrated to be only an effect of hydroxide formation as a result of the moisture content in the hydrogen gas applied in the storage tests. In 1991, Fan et al. and Cheng et al. reported on up to 13 wt% hydrogen storage capacity for tubular-shaped vapour-grown carbon nanofibers, whereas in graphitic nanofibers Gupta and Srivastava measured up to 10 wt% hydrogen, this range was confirmed by Browning et al. which found a hydrogen storage capacity of 7 wt% for carbon nanofibers [4]. Besides these optimistic publications, many other experimental results showed no hydrogen storage capacity at all in carbon nanofibers. Measurement of the hydrogen uptake with a microbal-
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V. Jiménez et al. / Chemical Physics Letters 485 (2010) 152–155 Table 3 Hydrogen storage publishing data of carbon materials obtained by different authors.
AC AC AC AC AC CNFs A-CNFs A-CNFs A-CNFs A-CNFs
T (K)
P (bar)
298 303 293 77 77 303 303 303 303 303
100 100 135 20 40 100 100 100 30 30
BET surface area (m 2/g)
900 800
R2 = 0.9553
700 600 500 400
R2 = 0.9493
300 200 100 0
0
2
4
6
8
10
12
20 18 16 14 12 10 8 6 4 2 0
H2 uptake (wt%)
Refs.
Activation conditions
0.85 0.67 0.90 5.00 5.50 0.35 0.42 0.33 0.53 1.03
[7] [8] [9] [8] [10] [11] [11] [11] [12] [12]
– – – – – Without activation KOH, 30 min. 900 °C, 100 ml/min Ar 100 ml/min, CO2, 30 min. 850 °C ZnCl2, 1050 °C, 120 min, 200 ml/min N2 KOH, 750 °C, 180 min, 200 ml/min N2
Cumulative micropore volume (cm 3/g)
Adsorption conditions
H2 absorbed (mmol/g) Fig. 3. Amount of H2 (absorbed at 77 K and 10 bar) as a function of BET surface area (- - -) and microporous volume ( ) for the different carbon materials. e: CNFs; hj: A-CNFs; N4: AC.
ance performed for vapour-grown nanofibers by Ströbel et al. yielded values below 1.5 wt%, this tendency was confirmed by Ahn et al. who measured values below 1 wt%. In the last few years, similar results have been published for several nanostructures. One possible explanation for the great discrepancies between the published values could be a strong dependence of the storage capacity on details of the carbon fiber synthesis and the activation process prior to the storage experiment [4]. On the other hand, natural and synthetic carbon can be activated following several different procedures to obtain microporous adsorbents showing very large specific surface area and considerable microporous volume. Reversible hydrogen uptake on these carbon was consistently reported to be approximately proportional to surface area and micropore volume, although the best linear correlation is usually obtained when relating hydrogen adsorption capacity to micropore volume, reflecting the fact that physisorption (and consequent hydrogen storage) in dominated by pores having a diameter in the subnanometer range. In fact, the interaction energy between hydrogen molecules and carbon adsorbents should be enhanced in narrow pores, because of overlap of the potential fields from both sides of the pore. Note that the hydrogen molecule has a kinetic diameter of 2.9 Å [13,14]. In Fig. 3 adsorbed amount of hydrogen at 10 bar and 77 K is plotted as a function of samples BET surface areas and microporous volumes. The linear behaviour (more accurate in the latter case) reported repeatedly in the literature for different porous systems (ordered porous carbons, activated carbons and activated carbon fibers) was observed, which points out to the crucial role of microporosity in H2 adsorption. As to the influence of graphitization degree, as evaluated by XRD, it seems that it does not favour H2 adsorption, in that more graphitic samples (CNFs) adsorb less hydrogen, whereas samples AC and A-CNFs adsorb more hydrogen. The linear relation between the micropore volume and the storage capacity
demonstrates that carbon materials ideal for hydrogen storage should possess a high microporosity with a small pore dimension. 4. Conclusions The hydrogen adsorption capacity of various carbon structures including activated carbon and carbon nanofibers has been measured as a function of pressure and temperature. Results have shown that, for all samples at room temperature and a pressure of 10 bar, the hydrogen storage capacity was less than 0.10 wt%. Upon cooling and pressurizing, the capacity of hydrogen adsorption increased obtaining the highest value (2.02 wt%) at 10 bars and 77 K. The correlation between hydrogen storage capacity and specific micropore surface area showed that the amount of H2 physisorbed depended almost linearly on it. The hysteresis in H2 uptake observed at 273 and 299 K principally with CNFs materials could be related with the pore window dimensions which is related to CNFs d002 values. Thus, if the cavities/pores are such enough wide (compared to H2 kinetic diameter) but not the windows to access to them, H2 will be loaded under high P but stored at low. Studied materials are still not practical storage materials but, however, the design of new CNFs to include thermally activated windows in the open structure could offer the possibility of modifying their desorption kinetics to improve their hydrogen storage characteristics. Acknowledgements The authors gratefully acknowledge financial support from Consejería de Ciencia y Tecnología de la Junta de Comunidades de Castilla-La Mancha (Projects PBI-05-038 and PCI 08-0020-1239). References [1] A. Züttel, Mater. Today 6 (2003) 24. [2] V. Jiménez, P. Sánchez, A. De Lucas, J.L. Valverde, A. Romero, J. Colloid Interf. Sci. 336 (2009) 226. [3] V. Jiménez, P. Sánchez, J.L. Valverde, A. Romero, J. Colloid Interf. Sci. 336 (2009) 712. [4] M. Rzepka et al., J. Phys. Chem. B 109 (2005) 14979. [5] M.O. Danilov, A.V. Melezhyk, G.Y. Kolbasov, J. Power Sources 176 (2008) 320. [6] X. Zhao, B. Xiao, A.J. Fletcher, K.M. Thomas, D. Bradshaw, M.J. Rosseinsky, Science 306 (2004) 1012. [7] H. Jin, Y.S. Lee, I. Hong, Catal. Today 120 (2007) 399. [8] W.C. Xu et al., Int. J. Hydrogen Energy 32 (2007) 2504. [9] P.-X. Hoy, S.-T. Xu, Z. Ying, Q.-H. Yang, C. Liu, H.-M. Cheng, Carbon 41 (2009) 2471. [10] M. Jordá-Beneyto, D. Lozano-Castelló, F. Suárez-García, D. Cazorla-Amorós, A. Linares-Solano, Micropor. Mesopor. Mater. 112 (2008) 235. [11] J.M. Blackman, J.W. Patrick, A. Arenillas, W. Shi, C.E. Snape, Carbon 44 (2006) 1376. [12] J.S. Im, S.-J. Park, T.J. Kim, Y.H. Kim, Y.-S. Lee, J. Colloid Interf. Sci. 318 (2008) 42. [13] K. Kadono, H. Kajiura, M. Shiraishi, Appl. Phys. Lett. 83 (2003) 3392. [14] A.W.C. Van den Berg, C.O. Areán, Chem. Commun. 2008 (2008) 668.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Hydrogen storage capacity on different carbon materials Vicente Jiménez a,*, Paula Sánchez a, José Antonio Díaz a, José Luis Valverde a, Amaya Romero b a b
Facultad de Ciencia Químicas/Escuela Técnica Agrícola, Departamento de Ingeniería Química, Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain Facultad de Ciencia Químicas/Escuela Técnica Agrícola, Universidad de Castilla-La Mancha, 13071 Ciudad Real, Spain
a r t i c l e
i n f o
Article history: Received 6 November 2009 In final form 9 December 2009 Available online 18 December 2009
a b s t r a c t The hydrogen adsorption capacity of various carbon structures including activated carbon (AC) and carbon nanofibers (CNFs) has been measured as a function of pressure and temperature. Results have shown the correlation between hydrogen storage capacity and specific micropore surface area and also explain the hysteresis in H2 uptake observed at 273 and 299 K principally with CNFs materials. The highest H2 storage value (2.02 wt%) was obtained with AC at 10 bar and 77 K. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
Hydrogen, as a futuristic energy resource free of air pollution and greenhouse effects (its combustion does not generate pollutants such as particles, nitrogen oxides, sulfur oxides, hydrocarbons and carbon monoxide), has attracted a great deal of attention in recent years. Thus, hydrogen storage is widely recognized as a critical enabling technology for the successful commercialization and market acceptance of hydrogen powered vehicles. The individual parts of hydrogen energy system such as production, delivery, storage and conversion are closely interrelated. Among them, hydrogen storage is one of the bottlenecks for the applications in automobile. Several storage methods have been investigated to develop efficient technologies [1]. Hydrogen storage by gas compression, low temperature liquefaction, metal hydride formation has several disadvantages. None of the current hydrogen storage technology satisfies all of the needs requested by end users and producers. Therefore, new investigations are centred in the development of novelty hydrogen storage materials. In this sense, it has been demonstrated that carbonaceous materials are attractive candidates due to their adsorption ability, high porosity, low mass density and low cost. In this work has been presented experimental results of hydrogen adsorption at different temperatures and pressures over activated carbon, carbon nanofibers and activated carbon nanofibers. Hydrogen adsorption data are interpreted according to the properties of the carbon materials (specific surface area, pore size distribution, etc.) and to the adsorption conditions (T, P).
2.1. Preparation and activation of the adsorbents
* Corresponding author. E-mail address: [email protected] (V. Jiménez). 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.12.026
Herringbone type carbon nanofibers were grown at atmospheric pressure in a fixed-bed reactor located in a vertical oven at a temperature of 600 °C according to the procedure described in [2,3]. Activated carbon was obtained from PANREAC. AC samples received the same HF treatment than synthesized CNFs for comparison purposes. CNFs were activated in an experimental set up consisting on a horizontal quartz reactor tube with a conventional horizontal furnace. Thus, the appropriate amount of CNFs was mixed with KOH and distilled water (10 ml water for 2 g KOH) and heated at 85 °C for 4 h under stirring and then dried for 12 h at 110 °C. Finally, the mixture was placed on a ceramic crucible located inside the horizontal reactor tube. The heat treatment consisted of a heating ramp from ambient temperature to 850 °C at a heating rate of 5 °C/min, followed by a 3 h plateau under 700 ml min1 He flow rate. Later, the system was cooling back to the initial temperature. The activated product (A-CNFs) was firstly washed with hydrochloric acid (5 M) to remove the activating agent and finally, with distilled water until a neutral washing was obtained. The resulting material was dried for 12 h at 110 °C in air to remove water prior to characterization.
2.2. Adsorbents characterization Surface area/porosity measurements were carried out using a Micromeritics ASAP 2010 sorptometer apparatus with N2 at 77 K as the sorbate. The samples were outgassed at 453 K under vacuum (6.6 109 bar) for 16 h prior to analysis; specific surface areas were determined by the multi-point BET method, pore geometry and size distributions were evaluated using the standard BJH
153
V. Jiménez et al. / Chemical Physics Letters 485 (2010) 152–155
treatment and micropore size distribution were evaluated using the Horvath–Kawazoe (H–K) equation. XRD analyses were carried out on a Philips X’Pert instrument using nickel filtered Cu Ka radiation; the samples were scanned at a rate of 0.02° step1 over the range 5 6 2h 6 90° (scan time = 2 s step1). This technique was used to evaluate the crystallinity of the carbon nanofibers.
cropore: 0.7–2 nm and ultramicropores: <0.7 nm); 2 nm < mesopore <50 nm and, macropore >50 nm. Obviously, AC sample presented the highest ultramicropore amount, followed by the A-CNFs sample. It can be observed that, the chemical activation process produced principally an increase in the ultramicropore volume (coincident with the BET surface area increase) although some supermicropores and mesopores were also generated. Note that although the quantity of mesopores increased as consequence of the activation, the percentage of mesoporosity inherent to the parent CNFs decreased, due to the high amount of ultramicropores created. Regarding to the pore development after an activation process, it is important to comment that different pore opening mechanisms exist. Whereas the micropore development is correlated with the partial opening of graphite layers in CNFs (d002 increase), the mesopore development is probably related to others behaviours such as, breaking of nanofibers or full exfoliation of graphite layers which can cause different sizes of mesopores according to the degree of exfoliation (even in the collapse of some micropore structures) [3]. The CNFs graphitization degree changes after the activation process can be quantified by means of npg values. After the activation, peaks corresponding to hexagonal (2h = 26°) and rhombohedral graphite (2h = 45°) become wider by treatment of CNFs with KOH (XRD patterns not showed), indicating that these CNFs were less graphitic in nature [2,3]. As consequence, d002 and Lc values increased and decreased respectively, being npg values lower than that obtained before the activation. Nevertheless, these npg values were pretty high if compared with those reported for conventional activated carbon or activated carbon fibers indicating that graphene layers remained undestroyed, and activation results in violation of parallel packing of layers (besides of burning some less ordered carbon) [5]. Table 2 shows the hydrogen adsorption at different temperatures and pressures obtained with the different carbon materials. Likewise, the variation of hydrogen uptake (mmol H2/g) at different temperatures and 10 bar of pressure are shown in Fig. 2. In the adopted experimental conditions, the following order in H2 uptake was observed: AC > A-CNFs > CNFs. With all carbon structures, hydrogen adsorption at 77 K resulted to be completely reversible, with almost no hysteresis phenomena and all isotherms having very similar shape. Nevertheless, it could be observed hysteresis in the adsorption–desorption kinetics above the supercritical temperature of H2, principally with CNFs materials. This behaviour
2.3. Hydrogen storage measurements Hydrogen adsorption capacities of carbon materials were measured using a Micromeritics ASAP 2050 (estimated error measurement ± 2%). The samples were outgassed at 453 K under vacuum (6.6 109 bar) for 16 h prior to analysis. The isotherm adsorption was obtained until 10 bar of absolute pressure to different temperatures. H2 storage at P > 10 bar was measured using a volumetric apparatus as explained in [4]. Prior to analysis, sample inserted into the cell was evacuated for several hours to a pressure level below 109 bar. 3. Results and discussion Table 1 summarizes the textural properties of the carbon materials as well as the principal XRD parameter, npg (mean number of grapheme planes in the crystallites). On the other hand, the proportion of ultramicropores, supermicropores and mesopores in the carbon samples are shown in Fig. 1. The following classification of the pores (IUPAC) was considered: micropore <2 nm (supermi-
Table 1 Textural properties and npg values of the carbon materials. AC
A-CNFs
CNFs
884 806 (91%) 17.276 (98%) 0.285 1.55
570 305 (54%) 3.431 (75%) 1.149 6.70
127 33 (26%) 0.147 (20%) 0.590 18.80
a
In brackets: percentage of micropore area with respect to the total surface area. Cumulative micropore volume obtained using the Horvath-Kawazoe method. In brackets: percentage of micropore volume with respect to the total pore volume. c Cumulative mesopore volume obtained using the BJH method. d Calculated using the formula npg = Lc/d002, where Lc is the average stacking height of carbon planes and, d002 is the average interlayer spacing. b
4.0
18
94 %
Parent CNFs Activated CNFs Activated carbon
3.5 66 %
14 12
2.5
3
3
Pore volume (m /g)
3.0
16
Pore volume (m /g)
BET surface area (m2/g) Micropore areaa (m2/g) Micropore volumeb (cm3/g) Mesopore volumec (cm3/g) npgd
10 2.0 8 1.5
25 %
1.0
6 4
9%
0.5 26 %
12 %
4%
62 %
2 2%
0.0
0 Ultramicropores
Supermicropores
Mesopores
Fig. 1. Proportion of ultramicropores (<0.7 nm), supermicropores (0.7–2 nm) and mesopores (2–50 nm) of the carbon materials (IUPAC classification).
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V. Jiménez et al. / Chemical Physics Letters 485 (2010) 152–155
Table 2 Hydrogen storage data of carbon materials. Pressure: 10 bar Temperature: 77 K mmol H2/g
wt%
Influence of the adsorption temperature AC 10.1 2.02 A-CNFs 5.0 1.00 CNFs 1.6 0.30 Pressure: 10 bar Temperature: 299 K Influence of the adsorption pressure AC 0.5 0.10 A-CNFs 0.2 0.05 CNFs 0.1 0.02
Pressure: 10 bar Temperature: 273 K mmol H2/m2
mmol H2/g
0.0130 0.0200 0.0500
0.0006 0.0008 0.0030
Pressure: 10 bar Temperature: 299 K mmol H2/m2
mmol H2/g
0.5 0.10 0.4 0.08 0.2 0.04 Pressure: 25 bar Temperature: 299 K
0.0006 0.0013 0.0060
0.5 0.10 0.2 0.05 0.1 0.02 Pressure: 35 bar Temperature: 299 K
0.0006 0.0008 0.0030
2.6 1.4 1.0
0.0030 0.0050 0.0300
3.2 2.0 1.4
0.0040 0.0080 0.0400
wt%
0.51 0.29 0.20
0.7 AC A-CNFs CNFs
10
0.6
2
Adsorbed hydrogen (mmol/g)
4
AC A-CNFs CNFs
273 K
0.4 Adsorbed hydrogen (mmol/g)
Adsorbed hydrogen (mmol/g)
6
0.65 0.40 0.28
mmol H2/m2
0.5 AC A-CNFs CNFs
77 K
8
wt%
0.5
0.4
0.3
0.2
299 K
0.3
0.2
0.1
0.1 0
0.0
0.0 0
2 4 6 8 10 Absolute pressure (bar)
0
2
4
6
8
Absolute pressure (bar)
10
0
2 4 6 8 10 Absolute pressure (mmHg)
Fig. 2. H2 adsorption–desorption isotherms at different temperatures and 10 bar for the carbon materials.
would allow H2 to be adsorbed at high pressures but stored at lower pressures. The differences in isotherm hysteresis must be related to the detailed characteristics of each porous structure [6]. CNFs and A-CNFs had the most marked isotherm hysteresis due to their smallest windows dimensions (d002) (similar to the 2.89 Å kinetic diameter of H2) combined with cavities(pores) that are much larger than H2 [3]. This fact impeded H2 access and at consequence, desorption of the trapped H2 guest to occur via window opening, which became noticeable at very low temperatures. As can be observed, at 299 and 273 K the hydrogen uptake (adsorption branch) was a linear function of the pressure indicating that no saturation in the investigated pressure range and that the adsorbed hydrogen layer on the carbon surface was very diluted. Nevertheless, at low temperatures (77 K) the isotherms can be fitted with a Langmuirtype equation (type I isotherm) indicating that saturation takes place with a hydrogen monolayer formation, as usual for microporous surfaces. In the absence of pore condensation effects (T > Tc) and relatively low P, adsorbents that have narrower pores (AC) maximize the interaction potential between hydrogen molecules and carbon adsorbents because of overlap of the potential fields from both sides of the pore, and lead to pore filling and superior performance at low P. Nevertheless, larger pores (CNFs/A-CNFs) allow increasing uptake at high P. Finally, notice that the hydrogen coverage per unit of micropore surface area at any pressure and/ or temperature was larger on nanostructures than on activated
carbon (last column of Table 2), which suggests that an increase of the surface area of these materials could possibly lead to interesting hydrogen storage capacities. As can be observed, excepting AC at 77 K, all the other experimented carbon materials did not allow any significant hydrogen storage. In spite of obtained results were very low, data were comparable with those of literature (see Table 3). In December 1996, Rodríguez and Baker claimed that their carbon nanofibers can store up to 70 wt% of hydrogen at room temperature and 140 bar hydrogen pressure. In recent years, different groups tried to reproduce the results of Rodriguez and Baker in similar or modified carbon nanostructures. It was claimed that alkali-doped carbon nanotubes can store up to 20 wt% of hydrogen, which later was demonstrated to be only an effect of hydroxide formation as a result of the moisture content in the hydrogen gas applied in the storage tests. In 1991, Fan et al. and Cheng et al. reported on up to 13 wt% hydrogen storage capacity for tubular-shaped vapour-grown carbon nanofibers, whereas in graphitic nanofibers Gupta and Srivastava measured up to 10 wt% hydrogen, this range was confirmed by Browning et al. which found a hydrogen storage capacity of 7 wt% for carbon nanofibers [4]. Besides these optimistic publications, many other experimental results showed no hydrogen storage capacity at all in carbon nanofibers. Measurement of the hydrogen uptake with a microbal-
155
V. Jiménez et al. / Chemical Physics Letters 485 (2010) 152–155 Table 3 Hydrogen storage publishing data of carbon materials obtained by different authors.
AC AC AC AC AC CNFs A-CNFs A-CNFs A-CNFs A-CNFs
T (K)
P (bar)
298 303 293 77 77 303 303 303 303 303
100 100 135 20 40 100 100 100 30 30
BET surface area (m 2/g)
900 800
R2 = 0.9553
700 600 500 400
R2 = 0.9493
300 200 100 0
0
2
4
6
8
10
12
20 18 16 14 12 10 8 6 4 2 0
H2 uptake (wt%)
Refs.
Activation conditions
0.85 0.67 0.90 5.00 5.50 0.35 0.42 0.33 0.53 1.03
[7] [8] [9] [8] [10] [11] [11] [11] [12] [12]
– – – – – Without activation KOH, 30 min. 900 °C, 100 ml/min Ar 100 ml/min, CO2, 30 min. 850 °C ZnCl2, 1050 °C, 120 min, 200 ml/min N2 KOH, 750 °C, 180 min, 200 ml/min N2
Cumulative micropore volume (cm 3/g)
Adsorption conditions
H2 absorbed (mmol/g) Fig. 3. Amount of H2 (absorbed at 77 K and 10 bar) as a function of BET surface area (- - -) and microporous volume ( ) for the different carbon materials. e: CNFs; hj: A-CNFs; N4: AC.
ance performed for vapour-grown nanofibers by Ströbel et al. yielded values below 1.5 wt%, this tendency was confirmed by Ahn et al. who measured values below 1 wt%. In the last few years, similar results have been published for several nanostructures. One possible explanation for the great discrepancies between the published values could be a strong dependence of the storage capacity on details of the carbon fiber synthesis and the activation process prior to the storage experiment [4]. On the other hand, natural and synthetic carbon can be activated following several different procedures to obtain microporous adsorbents showing very large specific surface area and considerable microporous volume. Reversible hydrogen uptake on these carbon was consistently reported to be approximately proportional to surface area and micropore volume, although the best linear correlation is usually obtained when relating hydrogen adsorption capacity to micropore volume, reflecting the fact that physisorption (and consequent hydrogen storage) in dominated by pores having a diameter in the subnanometer range. In fact, the interaction energy between hydrogen molecules and carbon adsorbents should be enhanced in narrow pores, because of overlap of the potential fields from both sides of the pore. Note that the hydrogen molecule has a kinetic diameter of 2.9 Å [13,14]. In Fig. 3 adsorbed amount of hydrogen at 10 bar and 77 K is plotted as a function of samples BET surface areas and microporous volumes. The linear behaviour (more accurate in the latter case) reported repeatedly in the literature for different porous systems (ordered porous carbons, activated carbons and activated carbon fibers) was observed, which points out to the crucial role of microporosity in H2 adsorption. As to the influence of graphitization degree, as evaluated by XRD, it seems that it does not favour H2 adsorption, in that more graphitic samples (CNFs) adsorb less hydrogen, whereas samples AC and A-CNFs adsorb more hydrogen. The linear relation between the micropore volume and the storage capacity
demonstrates that carbon materials ideal for hydrogen storage should possess a high microporosity with a small pore dimension. 4. Conclusions The hydrogen adsorption capacity of various carbon structures including activated carbon and carbon nanofibers has been measured as a function of pressure and temperature. Results have shown that, for all samples at room temperature and a pressure of 10 bar, the hydrogen storage capacity was less than 0.10 wt%. Upon cooling and pressurizing, the capacity of hydrogen adsorption increased obtaining the highest value (2.02 wt%) at 10 bars and 77 K. The correlation between hydrogen storage capacity and specific micropore surface area showed that the amount of H2 physisorbed depended almost linearly on it. The hysteresis in H2 uptake observed at 273 and 299 K principally with CNFs materials could be related with the pore window dimensions which is related to CNFs d002 values. Thus, if the cavities/pores are such enough wide (compared to H2 kinetic diameter) but not the windows to access to them, H2 will be loaded under high P but stored at low. Studied materials are still not practical storage materials but, however, the design of new CNFs to include thermally activated windows in the open structure could offer the possibility of modifying their desorption kinetics to improve their hydrogen storage characteristics. Acknowledgements The authors gratefully acknowledge financial support from Consejería de Ciencia y Tecnología de la Junta de Comunidades de Castilla-La Mancha (Projects PBI-05-038 and PCI 08-0020-1239). References [1] A. Züttel, Mater. Today 6 (2003) 24. [2] V. Jiménez, P. Sánchez, A. De Lucas, J.L. Valverde, A. Romero, J. Colloid Interf. Sci. 336 (2009) 226. [3] V. Jiménez, P. Sánchez, J.L. Valverde, A. Romero, J. Colloid Interf. Sci. 336 (2009) 712. [4] M. Rzepka et al., J. Phys. Chem. B 109 (2005) 14979. [5] M.O. Danilov, A.V. Melezhyk, G.Y. Kolbasov, J. Power Sources 176 (2008) 320. [6] X. Zhao, B. Xiao, A.J. Fletcher, K.M. Thomas, D. Bradshaw, M.J. Rosseinsky, Science 306 (2004) 1012. [7] H. Jin, Y.S. Lee, I. Hong, Catal. Today 120 (2007) 399. [8] W.C. Xu et al., Int. J. Hydrogen Energy 32 (2007) 2504. [9] P.-X. Hoy, S.-T. Xu, Z. Ying, Q.-H. Yang, C. Liu, H.-M. Cheng, Carbon 41 (2009) 2471. [10] M. Jordá-Beneyto, D. Lozano-Castelló, F. Suárez-García, D. Cazorla-Amorós, A. Linares-Solano, Micropor. Mesopor. Mater. 112 (2008) 235. [11] J.M. Blackman, J.W. Patrick, A. Arenillas, W. Shi, C.E. Snape, Carbon 44 (2006) 1376. [12] J.S. Im, S.-J. Park, T.J. Kim, Y.H. Kim, Y.-S. Lee, J. Colloid Interf. Sci. 318 (2008) 42. [13] K. Kadono, H. Kajiura, M. Shiraishi, Appl. Phys. Lett. 83 (2003) 3392. [14] A.W.C. Van den Berg, C.O. Areán, Chem. Commun. 2008 (2008) 668.